Multiple-effect centrifugation process and apparatus



y 8, 1951 H. w. SCHULZ 2,551,815

MULTIPLE-EFFECT CENTRIFUGATION PROCESS AND APPARATUS Filed Sept. 25,1945 LIGHT LEGEND PRODUCT I.HOUSING o. AxIAI TUBE 4 2. RoToR II. FEEDEFFECT 3. HOLLOW SHAFTING I2. RADIAL CHANNEL AIR 3 AIR 4. TURBINE l3.HEATER I 5. coNcENTRIo SHAFTING I4. CONDENSER e. INNER SHAFTING I5.CONDENSER FINS 7. ANNULAR RINGS I6. cow SURFACE 8. DISK-LIKE BAFFLES l7.STUFFING BOX 9. VERTICAL SPACERS 4/ SECTION B" 8 FIGURE 2 5 SECTION A-AFIGURE 3 W'% T INVENTOR ATTORNEY VERTICAL SECTIONAL VIEW OF CENTRIFUGEPatented May 8, 1 95i UNITED STATES FATENT QFFICE MULTIPLE-EFFECTCENTRIFUGATION PROCESS AND APPARATUS The subject of this invention is abasic improvement in the art of centrifugations, wherein a number ofsuccessive separations are accomplished in a single revolving unit, orin a multiplicity of revolving units. The invention is particularlyapplicable to an improvement in ultracentrifugation processes and theapparatus for the separation of fluids having small differences indensities and boiling points, such as isopotic modifications of acompound.

The ultracentrifuge, in one or" its most useful present embodiments,consists essentially of a revolving vessel containing the fluid mixtureto be separated. This vessel is rotated at high speed whereby the fluidsare subjected to a high centrifugal field of force, which tends toconcentrate the denser fraction of the fluid at the periphery of thevessel, and the lighter fraction at the axis, with material ofintermediate composition distributed according to a density gradient.The fluid to be separated may consist either of a liquid, vapor, or gas,or a combination of these. The revolving vessel may be supported by anair bearing or by suspension in a magnetic field. The cylinder may bedriven by means of a flexible shafting connected to an air turbine or toa highspeed synchronous motor. customarily, this shafting is hollow topermit introduction or removal of fluid from the vessel. Suitable ductsto remove fluid from the periphery of the vessel are provided and areconnected to the shafting intended for the removal of heavy fractions.The lighter fraction is removed from the axis of the vessel through asuitable shafting.

The rotating vessel is generally surrounded by a housing fitted withbearings through which the flexible shafting extends. The housing may beevacuated or filled with an inert gas at reduced pressure. The housingalso may provide a means for temperature control within the vessel.

Whatever may be the mechanical form of presently knownultra-centrifuges, they are subject to definite limitations. The degreeof separation in each operation is a function of the density differencebetween the fluids, the peripheral speed or" the centrifuge, and thetemperature. This degree of separation is termed unit separation factorand represents the maximum separation obtainable for a given densitydifference, speed of rotation, and temperature. As expressedmathematically for a two component mixture, it represents the ratio or"the concentration of the light component at the axis to theconcentration of light component at the periphery. The unit separationfactor for a gas or a 4 Claims. (Cl. 233-11) vapor may be computed bythe following theoretical equation:

V=peripheral speed R=gas constant T absolute temperature.

In the present method of centrifugation, the degree of separation in asingle centrifugation is limited to the unit separation factor, asdefined above. tor is inherently small, as in the separation of isotopicmodifications of a compound, a large number of centrifugations must becarried out to effect a substantial enrichment of the desired component.Thus for a unit separation factor of 1.04, it would requireapproximately 50 successive centrifugations to increase theconcentration of a desired component from one per cent to seven percent. Since for practical reasons it may be expedient to separate eachcharge into two equal fractions, the amount of material available foreach successive centrifugation is one-half that employed in the Previouscentrifugation. Thus,

in the example given above, to obtain even one miligram of the desired"I per cent material, it would be necessary to start with Whollyprohibitive quantities of the 1 per cent material, or perform a whollyprohibitive number of individual centrifugations in exhaustivereprocessing of intermediate products.

Thus, presently known centrifugation processes are wholly inapplicablefor separating material of extremely small difierences in density. Forinstance, the separation of the compounds of uranium from those ofuranium is a matter of economic and military significance, becauseuraniun'i is a potential source of atomic energy. However, the unitseparation factor of uranium hexafiuorid from uranium hexafluoride is ofthe order of 1.0%, under practical conditions of temperature andperipheral speed, so that its separation by present centrifugationmethods is quite impractical, as indicated above.

This invention provides a method of centrifugation, whereby a multipleeffect separation obtained by continuously passing material through aseries of centrifugation effects, unit separation fact-or beingapproximated in each ellect; the lighter fraction removed in each effectbeing passed continuously to a succeeding In cases where the unitseparation fac effect richer in a lighter component, and the heavierfraction removed in each effect likewise being passed continuously to apreceding effect richer in a heavier component. In particular, theinvention provides for the operation of a twophase system for thepurpose of accomplishing the transfer of material from on effect to anadjoining effect. In this manner, depending on the number of effectsinvolved, a high degree of separation may be obtained without handlingprohibitive quantities of materials, or performing a prohibitive numberof centrifugations. The invention particularly relates to an apparatusfor accomplishing multiple effect centrifugation within a singlerevolving unit. The invention as applied to an appropriate system willnow be more particularly discussed, but the process of this invention isnot limited to the particular apparatus described.

This apparatus is designed to separate a condensible vapor, both liquidand gas phases being present. The light component separatedin eacheffect passes as a vapor to the succeeding effect, and the heaviercomponent separated in each effect passes as a liquid to the precedingeffect.

The schematic construction of a suitable apparatus is shown in Figures1, 2 and 3. Figure 1 shows a vertical sectional view of the centrifuge,and Figures 2 and 3 represent vertical and horizontal sections of asingle centrifugation effect. The centrifuge consists of a housing i inwhich is suspended the rotor or spinning chamber 2. This rotor is drivenby means or a flexible, hollow shafting 3 connected to a turbine 4. Thefluid to be separated is fed through the center tube 6 of concentricshafting 5, which serves to position the rotor 2, but need not beconstructed as a thrust bearing, since the rotor is preferably suspendedin the housing as previously discussed. The light product is removedthrough the hollow shafting 3 and the heavy product through the annularspace of concentric shafting 5.

The interior construction of the rotor 2 is an important feature of theapparatus. The rotor is divided into a number .of horizontalcompartments, termed effects, by means of annular rings E and disk-likebaffles 3. These rings and baffles are supported, spaced and positionedby means of vertical spacers 9 which divide each effect intosector-shaped compartments by means of radially disposed vertical walls.

The annular rings 7 fit tightly against the wall of the revolvingcylinder and when the centrifuge is in operation constitute a damagainst the passage of the liquid phase along the periphery of thecentrifuge, except by overflowing the circular barriers. These ringshave a uniform inside diameter equal to the diameter of the liquid-gasinterface it is desired to maintain durin operation. In other words, theheight of the circular dam in the radial direction is equal to thethickness of the liquid film spread along th wall of the cylinder bycentrifugal action. In general, it will be desirable to maintain thethickness of the liquid film at a minimumin order to reduce the liquidhold-up of the system.

The disk-like baffles 8 are placed slightly above each circular ring andprotrude into the liquid phase during operation in such a manner thatthe liquid affords an effective seal against the passage of vapor fromone effect to the next along the liquid-gas interface, while at the sametime permitting the fiow of liquid past the baflie along the wall of thecylinder. This means that the '4 outside edge of the disk should extendapproximately half-way into the liquid film, except that it may extendto the wall of the cylinder at regular intervals for positioning andstructural support, as indicated in Figure 3. Each baffle is providedwith a small concentric hole through which the vapor may pass from oneeffect to the next. This system of rings and baffles restricts thepassage of fluid between the effects thus created to the movement ofliquid along the interface solely by overflowing the dams, and themovement of vapor solely along the axis of the revolving vessel.

The vertical spacers 9 consist of vertical walls which extend radiallyfrom a small concentric cylinder to which they are attached, to theperiphery of the rotor, thereby dividing each horizontal efiect into anumber of sector-shaped compartments. The supporting cylinder of eachspacer fits snugly around a small hollow tube I 9, Which extends alongthe axis of the centrifuge from end to end. The height of the verticalspacers may be determined by the number of effects it is desired toprovide for a given length of centrifuge. In general, the smaller theheight per effect, the greater the number of effects, but the smallerthe output of product for a given centrifuge. The spacers serve tosupport the rings and baffles, as indicated in the diagram. Theconcentric openings in the baffles may be larger in diameter than theoutside diameter of the supporting cylinders of the spacers to provide apassage for the vapors from effect to effect.

The radial walls of the vertical spacers that divide each horizontaleifect into sector-shaped compartments serve the additional purpose ofcounteracting the undesirable acquisition of angular acceleration by amolecule moving in a radial direction.

A feature of the above arrangement of rings, baffles and spacers is thatit admits readily of a piece by piece assembling of the interiorstructure of the centrifuge, and, more, particularly, that the numberand capacity of the single effects can be readily changed by usingspacers of different heights. The rings, baffies and spacers must be ofsymmetrical design and careful construction to balance weightdistribution about the axis. If preferred, the interior assembly may ofcourse be constructed as a rigid unit. The material of constructionshould be selected to withstand the stresses imposed and the corrosiveaction of the fluid to be processed.

The hollow axial shaft Iil serves as a channel for the introduction offeed to the particular effect H where the concentration of thecomponents is the same as that of the feed, marking the transition fromenriching to exhausting effects. The hollow section of tube Ill connectswith radial channels I2 incorporated into the radial walls of the spacerof the feed effect, so that the feed may be introduced at a point nearthe gas-liquid interface.

At the bottom of the rotor i provided an annular heating element i3adjacent to th wall of the rotor, which is heated by induced electricalcurrent or in some other manner. Alternative- 1y, heat may be radiatedfrom the housing to the rotor at this lower part. The function of theheat input is to vaporize all liquid overflowing the annular ring nextabove the heated section.

At the top of the rotorthere is provided a condenser M, whichaccomplishes the removal of heat from the top of the rotor. Thecondenser may have fins H5 in order to provide additional condensationsurface. The heat is conducted through the fins is through the top plateof the rotor and radiated from there to a cold surface 55 mounted in thetop of the housing. The temperature of the cold surface may bemaintained by refluxing a low-boiling liquid under constant pressurewithin a chamber in contact with surface it. However, other means ofcooling this surface may be provided. Stufiing boxes ii are provided asshown to afford necessary seals between stationary members and therevolving shafting.

The apparatus just described is adapted for the separation of two ormore fluids having the same or different boiling points but differing indensities. The fluid is introduced in the form of a vapor, and thetemperature within the rotor is maintained at or about the boiling pointof the fluid mixture under the pressure maintained at the gas-liquidinterface. As a consequence, both a liquid and a gas phase inequilibrium with each other are present in each effect throughout therotor. In operation, the liquid phase will be conoentrated along thewall of the rotor by means of centrifugal force, whereas by the samemeans, the gas phase will be distributed radially along a decreasingdensity gradient to the center of the rotor, with the lightest fractionat the center.

The system of rings, baffles and spacers just described will cause therotor to be divided into effects wherein the liquid phase of one effectwill be prevented from mixing with the liquid phase of another effect bymeans of the rings 7. Similarly, the vapor phase of one effect may passto the next effect only through the circular opening in the center ofeach baffle, because of the liquid seal provided between the rings andbafiies at the periphery.

In one method of operation, the feed will be introduced as a gas into aneffect near the middle of the rotor, wherein it passes through thechannels [2 in the vertical spacer to a point near the liquid-vaporinterface. As a consequence of centrifugal action, the les densefraction will tend to be concentrated as a gas at the center, whereasthe heavier fraction will gravitate to the gas-liquid interface. Here, apart of the heavier components in the vapor phase will condense and inso doing vaporize an equivalent amount of the lighter components fromthe liquid phase tending to establish vapor-liquid equilibrium at theinterface. This equilibrium is continuously disturbed in each effectboth by the influx of vapor richer in a lighter component from the nextlower effect in the series and by the overflow of liquid richer in aheavier component from the next higher effeet in the series, assubsequently described. The flow of feed is adjusted to the capacity ofthe centrifuge so that approximate conditions of equilibrium exist ineach effect. The lighter vapor will be continuously separated anddisplaced from the middle effect to the next higher effect. In thateffect it will be separated into a light and heavy fraction as in thepreceding effect. Thus, the vapor passing up through the passages at thecenter of the rotor will become progressively enriched in the lighterfraction. When the vapor reaches the top effect, part will be removed aslight product by means of a vacuurn pump, and part will be condensed andreturned as reflux. The condensation of part of the vapor in the topeffect will create a partial vacuum at the top of the rotor which willdraw the vapor column continuously up the central passage. In the courseof the upward flow of vapor from effect to effect, the heavier moleculeswill be continuously replaced by lighter molecules as a consequence ofthe centrifugal separation achieved in each effect.

The liquid condensed at the top of the rotor is termed the reflux andwill be immediately thrown to the periphery of the rotor, where it willcause an overflow of liquid across the annular ring separating thiseffect from the one immediately below it. This is repeated from effectto effect so that the liquid passing downward along the periphery willbe enriched in a heavier fraction. By constructing the baffles so thatthe extend nearly to the walls of the rotor, the liquid displaced fromeach effect will tend to be the heaviest fraction existing in thateffect. In the particular embodiment of the invention described, theeffects below the feed effect will serve as an exhausting section toconcentrate the heavier fraction of the feed, while those above the feedplate will serve as an enriching section to concentrate the lighterfraction of the feed. In the bottom effect, all the liquid is vaporized,a part thereof to be withdrawn as heavy product and the remainder to berecycled as reboil through the cycle described.

The above description has reference to operation with both enriching andexhausting sections in th centrifuge, but it is understood the feedcould be introduced either at the top or bottom, and the centrifugeserve either as an exhausting or enriching column. The form of the rotorneed not necessarily be cylindrical, but could for example be in theform of a truncated cone, and such an arrangement could accommodate avariable reflux ratio between effects by providing partial condensationin the effects.

For a given unit separation factor, the degree of separation will dependupon the number of effects provided and upon the reflux ratio employed.At total refiux, the over-all enrichment will be approximately the unitseparation factor raised to a power corresponding to the number ofXf=concentration of desired component in feed A=unit separation factor,and N number of effects In operating the centrifuge, the proper ratiobetween the withdrawal rates of both light and heavy products may bemaintained by selecting the cross-sections of the respective exit shaftsin that ratio, and maintaining identical pressures in both the light andheavy product receivers. The amount of feed may similarly be controlledby bleeding it into an intermediate low-pressure chamber maintained at acalculated pressure, from which the centrifuge will withdraw an amountautomatically equal to the sum of the light and heavy product by thedifferential Dressures created. The magnitude of the receiver pressurestogether with the rotational speed of the tube will determine thepressure at the gasliquid interface, and thus the temperature at whichthe system must be maintained in order to have the liquid at its boilingpoint.

The ratio of the fraction returned to the system as reflux to thefraction withdrawn a light product, known as the reflux ratio, has amini mum value-knownas the minimum reflux ratio, as fixed-by therequirement of a material balance under steady state operation. Thminimum reflux ratio is a function of the unit separation factor and ofthe ratio of the desired concentration of light component in the productto the concentration of light component in thefeed, and is given by theequation:

where L=minimum reflux ratio X =concentration of desired component inlight product Xr concentrationof desired component in feed A==unitseparation factor.

In practice it is desirable to use a reflux ratio greater than thetheoretical minimum in order to obtain a greater over-all separation fora given number of effects. However, the production capacity of thesystem its-correspondingly, decreased until under total reflux noproduct can be removed.

The followin operating conditions are recommended for the separation ofU Fc and U Fs, assuming that it be desired to concentrate U Fe from aconcentration of 0.71% in the feed to a concentration of in the product.The ten.- perature of operation is 70 the peripheral speed is 27,000cur/sec, and the unit separation factor, is 1.039. Under theseconditions, the minimum reflux ratio is 154 to l, and the number oftheoretical effects required in the enriching section when operatingunder total reflux is 50.

Feasible dimensions for the rotor of the centrifuge are a length of sixfeet and a diameter of four inches, if operation is intended on alaboratory scale.

It is to be understood that the attached drawings are schematic toillustrate the principle of the invention since the general constructionof the ultracentrifuge with its necessary bearings and shafts forintroducing and withdrawing products have already been described in theliterature.

I claim:

1. Process for separating or concentrating in the presence of bothliquid and vapor phases a mixture of fluids of different densities andsubstantially the same volatilities, which comprises successivelysubjecting said fluids to a multiplicity of independent centrifugationeffects, wherein the liquid phase is centrifugally concentrated in alayer having a contour determined by the rotation of a symmetricalgeometrical figure about its axis of symmetry, 1e centrifugal force ineach effect being sufiicient to distribute the components of the vaporphase along a density gradient from the axis of rotation to said layer,causing a fraction richer in heavier component to condense in said layerin each effect, causing the liquid phase to flow from eiiect to effectin the direction of an increasing concentration of heavier component,causing the vapor phase to flow from effect to effect in the directionof an increasing concentration of lighter component along a pathsurrounding and including said axis of rotation, maintaining thetemperature in each effect substantially the same as the boiling pointof the fluid mixture under the pressure maintained at the vapor-liquidinterface, supplying heat to .an efiect having the highest concentrationof heavier component and vaporizing at least some of the liquid phasepresent therein, and removing heat from an effect having the highestconcentration of lighter component, and condensing at least some of thevapor phase resent therein, the amounts of heat supplied and Withdrawnbeing sufficient to cause said liquid and vapor phases to flow fromeffect to effect in the directions stated.

2. Process as claimed in claim 1 in which the fluid mixture to beseparated or concentrated is continuously introduced as feed to aneffect having a concentration of components similar to that of the feed,and in which products are continuously Withdrawn, the product-enrichedin a lighter component being withdrawn from an effect disposed fromthefeed effect in the direction of increasing concentration of lightercomponent, the product enriched in-a heavier component being-Withdrawnfrom an effect disposed from the feed effect in the direction ofincreasing concentration of heavier component.

3. An ultracentrifuge in which mixtures of fluids are separated orconcentrated in the presence of both their liquid and vapor phasescomprising a rotor supported for rotation about a vertical axis in ahousing, said rotor being divided into-multiplicity of sections by meansof rings and circula shaped baffles; said rings being fitted against thewall of the rotor and said bafiles extending past the innercircumference of said rings but not to the wall of the rotor throughouttheir entire corcumference, thus effooting a liquid seal along the wallof the rotor in cooperation with liquid held against the wall by therotation of the rotor, the baiiies having an openin radially disposedfrom the axis of the rotor to permit separated lighter vapor to passfrom section to section, said ultracentrifuge being equipped with meansfo Withdrawing heat near the top of the rotor, means for introducingheat near the bottom of the rotor, and means for introducing andwithdrawing fluid to and from the rotor.

4. An ultracentifuge in which mixtures of fluids are separated orconcentrated in the presence of both the; liquid and vapor phasescomprising a cylindrical rotor supported for rotation about a verticalaxis in a housing, means for driving said rotor at peripheral speedsoperative to concentrate the liquid phase on the wall of said rotor andthe vapor phase along a density gradient from the axis of the rotor,said rotor havingcentrally located hollow shafting for introducing andwithdrawing fluid to and from the rotor, and being divided into sectionsby means of removable horizontal discs fitted around said shaftingthrough a concentric opening and extending at spaced intervals to theWall of the rotor, said discs having a plurality of circumferentialopenings, and said concentric opening to permit separated lighter vaporto pass from section to section, in conjunction with said discs, solidrings fitted against the wall of the rotor and having an innercircumference closer tothe axis than said circumferential openings, saidultracentrifuge containing cooling means at the top of the rotor forcondensing vapor as reflux in said rotor, and heating means at thebottom of the rotor for evaporating liquid in said rotor.

HELMUT W. SCHULZ.

(References on following page) REFERENCES CITED The following referencesare of record in the file of this patent:

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